It’s easy to believe that when Abraham Lincoln drafted his second inaugural address in 1865 — writing that if God willed it, the struggle against slavery would continue until “every drop of blood drawn with the lash shall be paid by another drawn with the sword” — he had in mind a famous photograph made two years earlier. The image shows a former slave, known only as “Gordon,” who had escaped bondage in Louisiana to freedom behind Union lines at Baton Rouge. The man appeared seated, with his face and torso turned away from the camera, showing a gruesome and abstract welter of lash marks and lacerations on his back.

The photograph was one of the most convulsive images of the 19th century, circulated widely by abolitionists, reproduced and disseminated not just through popular magazines but on visiting cards, small reproductions on card stock that could be purchased and collected in albums. As Frank Goodyear writes in a recently published Smithsonian book, “Photography Changes Everything,” the image not only galvanized antislavery sentiments, but it “also inspired many free blacks in the North to enlist.”

The photograph appears in a small National Portrait Gallery exhibition, “Bound for Freedom’s Light: African Americans and the Civil War,” which, curator Ann Shumard said, is designed “to drive home the point that African Americans weren’t simply passive observers on the sidelines of this conflict.” The show includes photography, engravings, bookplates and drawings, and occupies a niche gallery sandwiched between two larger long-term displays of Civil War material.

The modest nature of the display contrasts sharply with the sumptuous oil paintings and traditional sculpture in the main galleries, a visual analog to the complicated way in which African Americans were caught up in the war, yet marginal to its outcome and direction. They fought, and they were fought over, but it was white men who wore the epaulettes, called the shots and ended up being memorialized in full-length, heroic portraits of power.

The exhibition inevitably focuses on how white people defined the parameters and meaning of African American existence. Opening with Gen. Benjamin Franklin Butler, a Union leader who had to grapple with the status of slaves fleeing the Confederacy to Union protection, the exhibition recalls one of the pivotal moments of the Civil War. In 1861, three slaves escaped to Union-held Fort Monroe in Hampton, Va., where Butler was in command. When their Confederate owner demanded their return, citing the noxious 1850 Fugitive Slave Act, Butler invented a neat legal solution: Defining them as “contraband of war,” the Union general refused to hand them over, but at the cost of keeping them in a dubious existential category.

“Butler became very defensive of what came to be known as his contraband,” Shumard says.

The slave Gordon also arrived as “contraband” but then enlisted in a black Union regiment. A page from the July 4, 1863, issue of Harper’s Weekly shows his transformation. On either side of a reproduction of the infamous image of his wounded back are before-and-after images showing him as he arrived, ragged, to the safety of Union protection, and later neatly dressed in his military uniform. Several of the most fascinating images in the exhibition underscore similar ideas of transformation, the malleability of character and appearance, as though African Americans are not only emerging as a new subject for photography, but morphing even as the lens tries to pin them down.

An engraving of the Emancipation Proclamation has an elaborate border decoration depicting the gulf between slave and free culture, contrasting images of whipping, human auctions and barren landscapes with popular Republican tropes of small farmers, prosperity and self-determination. A rare, large-format print of a group of emancipated slaves who toured the North to raise funds for schools shows three adults and five children, all of them smartly dressed. The boys stand with one hand tucked into their buttoned jackets; the girls are in crisp dresses cinched snugly at the waist. But clearly visible on the forehead of one of the men is a brand mark, one of the more brutal ways in which white owners imprinted identity onto the bodies of black slaves.

Thus, slavery appears in the exhibition mainly as it is written onto the body, through whips and brands. Otherwise, African Americans become a subject for photography only as they make exceptional efforts at self-empowerment, either through escape, enlistment or other aid to the Union cause. Two delicate tintype images, contained in elaborate folding boxes, show African American men in Union uniform, standing smartly for a standard-issue portrait. Another image shows what must have been a shockingly, or thrillingly powerful woman, Harriet Tubman, holding a gun and looking very much not-to-be-messed-with.

But it is Gordon who remains the most indelible presence. It isn’t easy, even today, to look square on at the image and acknowledge the depravity that caused his suffering. He is a statuesque presence, and with some kind of clothing gathered around his waist, it almost seems as though he was posed to recall a Renaissance statue. The image reminds one of the hooded man with his arms in the crucifix position that emerged from the American torture cells of Abu Ghraib in 2004. Its aesthetic power is a mix of the accidental echo of old artistic forms and the pure horror of what it depicts.

If Lincoln didn’t have it on his desk when he reminded Americans of the terrible moral debt incurred by 250 years of slavery, he must certainly have had it in his mind’s eye.

Bound for Freedom’s Light: African Americansand the Civil War

is on view through March 2, 2014, at the National Portrait Gallery, Eighth and F streets NW. 202-633-1000. www.npg.si.edu.